In 1.26 there is only health indicator, the hearts. In 1.27 there will be three: health, hunger and outside temperature. Have a look:
The temperature indicator shows environment temperature, not body heat like in the vital stats panel.
When it’s all blue, it means the temperature is optimal. When it turns dark, it means it’s too cold. When it turns orange, it’s too hot.
If you are properly clothed, with long johns, socks, sweaters etc., you can function in almost all temperatures. Only the most extreme cold, when the indicator is all dark might make you shiver a little. Unless you get wet. Being wet pretty much nullifies all clothing until you get it dry. In 1.27 you can stand close to a campfire to do so.